1. Field of Invention
The present invention relates to a method and apparatus, particularly suitable for use in ink-jet printing apparatus, for minimizing printing defects which are caused by ejecting smaller than desired drops.
2. Description of Related Art
Liquid ink printers have at least one print-head from which droplets of liquid ink are directed towards a recording medium. Various types are known. Typically, they are of a continuous stream or a drop-on-demand type. Examples include piezoelectric, acoustic, micro-electro-mechanical systems (MEMS), phase change wax-based, or thermal ink printers. Within the print-head, the ink is contained in a plurality of ink conduits or channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the ends of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a heat transducer or a resistor, typically individually addressable and associated with one of the channels. As voltage is applied across a selected resistor, the temperature of the ink in the associated channel rises until some of the ink transitions from liquid to vapor. The vapor bubble expands in size, pushing a stream of ink out of the channel orifice, and then contracts as it cools, retracting the ink in the channel and pinching off the ink stream. Thus a droplet of ink is formed moving in a direction away from the channel orifice and towards the recording medium. Upon hitting the recording medium, a dot or spot of ink is deposited. The channel is then refilled by capillary action from a supply container of liquid ink.
The ink jet print-head may be incorporated into a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small print-head containing the ink channels and nozzles. The print-head can be functionally attached to a disposable ink supply cartridge and the combined print-head and cartridge assembly is attached to a carriage, which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or transparencies. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. When there are large blocks without data to print, the paper may be stepped a larger amount. This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary print-head having a length sufficient to print across the entire width or length of the recording medium at a time. The recording medium is continually moved past the page width print-head in a direction substantially normal to the print-head length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
A commonplace problem in inkjet printing technology is proper placement of ink on the paper. There is always some non-uniformity of drop volume, which can depend on the particular nozzle geometry, the temperature of the print-head, surface tension and viscosity variations in the ink, the local fluid dynamics (which are largely determined by the printing history), the power applied, and many other factors. Some of these variations are well understood, and one can make a systematic adjustment to reduce the variability. For example, using different driving pulses at different temperatures results in a fairly uniform drop volume over an extended temperature range.
One mechanism behind non-uniform drop volume is fairly well understood. Thermal inkjet print-head nozzles tend to lose water vapor or other solvents through evaporation at the open orifice openings (i.e., latency). This causes an increased ink viscosity and other changes in the ink physical properties at or near the nozzle orifices, and may also change the size and speed of subsequently ejected drops. The particular ink formulation depends on the amount of water or other solvents lost, which in turn depends on the amount of time the ink is exposed to the atmosphere, the humidity, the nominal ink formulation, the nozzle geometry, and other factors. In a severe manifestation of latency, a significant quantity of water or other solvents is lost, and a viscous plug form at the nozzle opening. It may be difficult or impossible to eject ink through this plug, and as a consequence, one or more drops may be missing from the printed image. Some ink formulations are more susceptible to this phenomenon than other, forming ink plugs in a second or less. Other ink and print-head combinations may form plugs at 60 seconds or longer.
In attempting to solve this problem, maintenance routines such as spitting into a waste bucket or flowing ink through the print-head with pressure or vacuum (i.e. priming) may be employed on a regular basis. However, these remedies are wasteful of ink and also can lower the speed and throughput of the printer. As inkjet performance surpasses three seconds per page, even less time is available for nozzle maintenance.
In attempt to solve this and related problems, people have tried to compensate for misfiring jets by determining the probability of misfire from each individual nozzle at the start of life under prescribed test conditions, irrespective of root cause. U.S. Pat. No. 6,238,112 to Girones et al. describes a method for revising the original print mask based upon which nozzles are more likely to fire based upon a statistical probability obtained during a test period.
U.S. Pat. No. 6,042,211 to Hudson et al., (hereinafter Hudson) describes a method to compensate for ink drop volume variance by modifying the CMYK tone levels. In Hudson, the drop volume at the start of life under prescribed test conditions is determined and stored on the print head/cartridge. If the print-head is found to have high or low nominal drop volume, then the data is used to increase/decrease the drop volume to the print-head by either adjusting the color lookup tables or modifying the voltages to the print-head. The method in Hudson applies to the complete print-head and not to a particular jet.
There is a need for improved methods and apparatus to compensate for drop volumes that vary due to dynamic changes in the local ink formulation.
Since it is not only possible to keep track of the time each nozzle has been left unfired, but also practical to do so because of current computer processing speeds, it is possible to forecast which pixels are likely to be small or missing.